Insulating coating



United States Patent INSULATING COATING Mauritz L. Anderson, Arlington,Mass., assignor to Raytheon Manufacturing Company, Newton, Mass., acorporation of Delaware 7 Claims. (Cl. 117--231) This invention relatesto insulating coatings for condoctors and more particularly to such acoating of alumina in a binder containing both aluminum nitrate andaluminum hydroxide.

It is customary to coat a heater wire for an indirectly heated cathodeinan electron or gaseous discharge tube with a layer of insulatingmaterial. One such coating in common use is made largely of aluminasuspended in a binder of aluminum nitrate, methanol and water. Whilesuch a coating is satisfactory for use in, for example, home receivertubes, there are certain other uses Where the tubes are exposed tounusual vibrations and shock. Under such use the aluminum nitratecoating does not adhere sufliciently to the wire of the heater, andbreaks in use to short. the heater to the cathode, thus making the tubeuseless.

It has also been found desirable to. have as smooth a finish as possibleon the outer surface of the coated conductor for ease in inserting theheater into the cathode sleeve. It is also desirable to expose as littleas possible of the conductor when the coated wire is folded to form aheater, for instance.

it has been found that these desirable qualities are obtained in greaterdegree in the coating of the present invention comprising alumina of. anaverage particle size of four microns, or less, in a binder includingaluminum hydroxide. as. well as aluminum nitrate.

The binder for such a coating is made in one of two ways. By the firstmethod, aluminum nitrate is reacted with ammoniumhydroxide to. producealuminum hydroxide as a. precipitate. WhlClllS. separated out andreacted with an insufiicient amount of nitric acid to: completelyneutralize the aluminum. hydroxide to give a mixture of aluminumhydroxide and aluminum nitrate which, with added water andmethanol,forms the binder.

By the second method: of making thebinder, aluminum nitrate is reactedwith. sufiicient ammonium. hydroxide to produce the amount of aluminumhydroxide that it is desired to have in the finished batch of binder.To' this the desired quantity of aluminum nitrate is added.

To this binder prepared by either of these methods, alumina of anaverage particle size of four microns, or less, is added. This forms asmooth adhering coating for the conductor. This coating, when appliedand baked, presents a smooth, silky finish such that a filament wirecoated with it may be inserted into a cathode sleeve with considerablygreater ease than the old coating of the aluminum nitrate type.

The presence of the aluminum hydroxide in the binder causes the coatingto adhere considerably more firmly to the conductor than coatingscontaining only aluminum nitrate. In comparative tests with tworepresentative aluminum nitrate coatings, hereinafter referred to ascoatings A and B, the following results were obtained. It requiredeighty grams of pull on a razor blade to strip a coating of the A typefrom a conductor, and 160 to 200 grams of pull to strip a coating of Btype, while it required 230 grams of pull to strip a coating madeaccording to the present invention.

The coating of this invention also exhibits considerably greaterresistance to crushing than the older type coatings. In the testreferred to above, it required 1,530 grams of steadily applied load tocrush a span of A coating one-eighth inch in width between two jaws. Itrequired 2,940 to 3,360 grams to crush such a span of B coating, but itrequired 5,800 grams to crush such a span of the coating of the presentinvention.

A characteristic batch of the binder of the invention is prepared from481 grams of aluminum nitrate dissolved inv two liters of distilledwater. 200 milliliters of ammonium hydroxide, NH4OH, of reagent gradecontaining 29.2 per cent. of ammonia (NH3), and having a specificgravity of 0.89, isadded slowly to this solution. Additional amounts ofammonium hydroxide are added in increments of about ten millilitersuntil the odor of ammonia gas is detected, indicating that the aluminumhydroxide produced by the reaction has been completely precipitated.With the quantities given, 253 milliliters of the ammonium hydroxidewould be required to completely convert the aluminum nitrate. However, aslight excess is desirable so that a total of 260 to 265 milliliters ofammonium hydroxide should be added to this batch to produce 100 grams ofdry aluminum hydroxide. The precipitated aluminum hydroxide is in amoist state. The quantities indicated produce 750 grams of moistaluminum hydroxide.

345 grams of nitric acid of per cent. purity would be required tocompletely neutralize 100 grams of aluminum hydroxide. This isequivalent to 243 milliliters of acid. However, as it is only desiredvto convert 78 per cent. of the hydroxide to aluminum nitrate leaving 22per cent. of the original quantity as aluminum hydroxide, only 190milliliters of acid are required.

After the acid is added, the mixture is heated, preferably in. a steambath, for several hours with occasional stirring until the interactionis complete. The resulting milky opalescent. solution is cooled to about25 degrees centigrade and water. added, if necessary, to give thesolution a specific gravity of 1.16 and a pH- value of 2.75 to 2.85.Additional acid may be needed. to attain this range of pH values..

The resulting binder comprises:

Percent by weight This binder can be produced by the alternative processdescribed below. In a representative batch 212 grams of aluminumnitrate, Al(NO3)3'9H2O, are dissolved in 100 milliliters of distilledwater and about 120 milliliters of ammonium hydroxide, NH4OH, added toreact with the aluminum nitrate to produce aluminum hydroxide by thesame process as above. The precipitated wet aluminum hydroxide isfiltered from the solution and dried, preferably by suction. To theresulting wet cake, 750 grams of aluminum nitrate, Al(NO3)s-9H2O, andone liter of water are added to the mixture which is then heated toabout to degrees centigrade. The aluminum hydroxide dissolves partiallyin the natural acidity of the aluminum nitrate salt to form asemi-colloidal material that is the same as the binder produced by thefirst method and has the same composition, specific gravity and pH.

One volume of methanol, (CI-IaOH), is added to three volumes of thesolution to produce the final binder comprising:

Percent by weight Aluminum nitrate 18.47 Aluminum hydroxide 1.88 Water61.37 Methanol 18.28

To produce the desired coating, sufficient alumina, A1203, of an averageparticle size of not more than four microns is added to produce amixture of the following composition:

Percent by weight Alumina 61.77 Aluminum nitrate 7.06 Aluminum hydroxide0.72 Water 23.46 Methanol 6.99

In production, it is frequently found that it is easier to apply thecoating if it is made somewhat thinner than indicated above. However, ifmore than 20 percent by volume of liquids is added, the result will betoo thin to be effective.

When the coating mixture is applied to the conductor and baked, themethanol and most of the water evaporate. During the baking at atemperature between 550 degrees centigrade and 850 degrees centigrade,the aluminum nitrate breaks up into aluminum oxide and nitrogentrioxide. The aluminum hydroxide also breaks up into aluminum oxide andwater.

The reason for using aluminum nitrate and aluminum hydroxide instead ofonly alumina is that alumina is not freely soluble in water while thenitrate is, and the hydroxide forms a colloidal suspension. Anotherconsideration is that a salt must be used that does not break down intoa by-product in addition to alumina that has a deleterious effect on thetube components. Chlorides and sulphates would give by-products onbaking that would have such deleterious effects.

What is claimed is:

1. An insulating coating composition for a conductor consisting ofapproximately sixty-two percent by weight of alumina in approximatelythirty-eight percent by weight of a binder consisting essentially ofaluminum nitrate, aluminum hydroxide, methanol and water.

2. An insulating coating composition for a conductor consisting ofapproximately sixty-two percent by weight of alumina with approximatelythirty-eight percent of a binder consisting of twenty percent aluminumnitrate, two percent aluminum hydroxide, sixty percent water, andeighteen percent methanol.

3. A binder for an insulating coating for conductors consisting oftwenty percent aluminum nitrate, two percent aluminum hydroxide, sixtypercent water and eighteen percent methanol, all by weight.

4. In the process of producing an insulated electrical conductor, theimprovement which consists of coating the conductor with a suspension ofalumina in a binder of twenty percent by weight of aluminum nitrate, twopercent aluminum hydroxide, sixty percent water and eighteen percentmethanol, heating to a temperature of between 550 and 850 degreescentigrade to adhere the aluminum compounds to the conductor and tothemselves and cooling to room temperature.

5. In the process of producing an insulated electrical conductor, theimprovement which consists of coating a conductor with an aqueous andmethanol suspension of alumina compounds in the following proportions:

Percent by weight Alumina 89 Aluminum nitrate 10 Aluminum hydroxide 1heating to a temperature of between 550 and 850 degrees centigrade toadhere the aluminum compounds to the conductor and to themselves andcooling to room temperature.

6. In the process of producing a binder for an insulating coating ofalumina, the improvement which consists of reacting aluminum nitratewith ammonium hydroxide to produce aluminum hydroxide, reacting thealuminum hydroxide with twenty-two percent by weight less than theamount of nitric acid necessary to neutralize the aluminum hydroxide toproduce a mixture of one part by weight of aluminum hydroxide to tenparts of aluminum nitrate and adding enough water and methanol toproduce a mixture of ten parts of aluminum nitrate, one part of aluminumhydroxide, thirty parts of water, and nine parts of methanol.

7. In the process of producing a binder for an insulating coating theimprovement which consists of the steps of dissolving aluminum nitratein water, adding ammonium hydroxide to produce aluminum hydroxide andammonium nitrate, reacting the aluminum hydroxide with twentytwo percentby weight less nitric acid than is sutficient to neutralize all thealuminum hydroxide to produce a mixture of one part by weight ofaluminum hydroxide to ten parts of aluminum nitrate and adding enoughwater and methanol to produce a mixture of ten parts of aluminumnitrate, one part of aluminum hydroxide, thirty parts of water and nineparts of methanol and having a specific gravity of 1.16 and a pH valueof between 2.75 and 2.85

References Cited in the file of this patent UNITED STATES PATENTS2,307,018 Cardell Ian. 5, 1943 2,530,546 Snyder Nov. 21, 1950 2,635,995Swanson Apr. 4, 1953 FOREIGN PATENTS 987,628 France Aug. 16, 1951 OTHERREFERENCES Hackhs Chemical Dictionary, 2nd ed., 1937, pp. 237- 238.

1. AN INSULATING COATING COMPOSITION FOR A CONDUCTOR CONSISTING OFAPPROXIMATELY SIXTY-TWO PERCENT BY WEIGHT OF ALUMINA IN APPROXIMATELYTHIRTY-EIGHT PERCENT BY WEIGHT OF A BINDER CONSISTING ESSENTIALLY OFALUMINUM NITRATE, ALUMINUM HYDROXIDE, METHANOL AND WATER.